This invention relates generally to a method of collecting or removing particles such as dust or smoke and other pollutants from a gas stream and particularly relates to a method of removing either particles or fluids from a gas stream by means of charged droplets.
Due to the increasing concern for ecology, it has become necessary to provide devices or methods for removing dust, smoke, and other fluid or solid pollutants from an air stream or smoke stack. Among such devices are electrostatic precipitators and so-called wet scrubbers.
The oldest and most common industrial precipitator is the basic Cottrell electrostatic precipitator. Here a corona discharge is passed through the space through which the gas stream flows. Accordingly, the particles in the gas stream are electrically charged by the accumulation of ions. The charged particles are subsequently attracted to collector plates where they are deposited and eventually removed. This electrostatic precipitator normally removes pollutant particles in the range from 0.1 to 10 micron diameter having a resistivity of 10.sup.6 to 10.sup.12 ohms-cm. Since the charge of the particle is caused by ions, the magnitude of the charge is limited because the number of ions is limited by the electrostatic field.
A number of design modifications have been attempted to effect an improved collection eficiency. One such device is exemplified by U.S. Pat. No. 1,958,406 issued to Darrah, which suggests the desirability of producing a fine spray of small diameter droplets. The apparatus to Darrah is characterized by a ring-shaped conductor or electrode positioned near a fluid discharge nozzle. This conductor is frequently referred to as an extractor plate. The purpose of an extractor plate is to impart an electrostatic charge to the fluid droplets and enhance the velocity thereof. The droplets are ejected under pressure from the fluid nozzle.
Other typical art devices are shown in the U.S. Pat. to Penney, Nos. 2,357,354 and 2,357,355 and Marks, No. 3,503,704. As is typical of most prior art devices, Penney and Marks also use an extractor electrode or electrodes which induce a charge on the droplets. A further characteristic of these devices is the fact that the charged droplets and particulates are carried downstream where the driving mechanism toward the wall electrodes is the space charge effect and relatively weak associated field which cause the droplets to move toward the walls.
A further characteristic of the prior art devices utilizing an extractor plate is the introduction of droplets at high velocity followed by a decrease in velocity as the droplet traverses the gas medium. This will occur since the electrostatic field is between the fluid nozzles and extractor electrode and the charged particles tend to return upstream along the flux lines; however, the gas stream carries them downstream. This variation in sweep rate of the droplet tends to produce a variation in the cleansing qualities from one region of the gas stream to another, and in general provides for a relatively slow sweep rate and small differences in relative velocities between the droplets and the particulates.
The British Pat. No. 5051 issued on Feb. 26, 1914 discloses a wet scrubber using a counter current flow of the gas stream and the droplets. The droplets are generated by a series of jets for producing a fine water spray. The gas flows through a vertical tube and therefore the water jets are also disposed in a circular arrangement. The water is charged negatively and the walls positively. The droplets are sprayed under pressure. Furthermore, the droplets in the interior of the water jets are shielded by the outer jets from the electrostatic field. This patent, therefore, fails to teach the generation of small size droplets by electrohydrodynamic action.
A paper by Hendricks, Jr. et al. entitled "Photomicrography of Electrically Sprayed Heavy Particles" which appears in the Journal of the American Institute of Aeronautic and Astronautics, Volume 2, No. 4, April 1964, pages 733 - 737 deals with electrostatic thrust devices for space flight. It is proposed in the paper to generate liquid drops by the application of an electrostatic field. However, the liquid is ejected into a vacuum where conditions are totally different from those where a liquid is ejected into the atmosphere. Thus in the first place the space charge surrounding a capillary is vastly different for vacuum conditions than for atmospheric pressure. Additionally, it has been found by extensive test and research that the motion and dispersal of charged droplets under vacuum conditions discussed in the Hendricks Jr. et. al. paper cannot be extrapolated with useful results under atmospheric conditions. It is also indicated that spraying of the liquid will occur around the periphery of the capillary through which the liquid is ejected.
The U.S. Pat. to Gilman, No. 2,525,347 also discloses an electrostatic apparatus where liquid droplets are generated. Here the gas stream which flows in the same direction as the droplets in initially ionized by an ionizing wire. This, of course, will charge the particles carried by the gas stream.
The liquid is sprayed out of a tube or nozzle and the droplets are accelerated by a ring electrode or extractor plate which is maintained positive while the nozzle and the walls are maintained at ground potential. Normally the droplets would be accelerated and collected by the extractor electrode. In order to avoid this it is either necessary to force the droplets to flow at such a velocity that they are carried past the extractor electrode or else to use a gas stream of sufficient velocity to prevent the droplets from being collected by the ring electrode. This, of course, imposes a severe restriction on the parameters of the charged spray apparatus. It will also be noted that there is zero relative motion between the particles of the gas stream and the droplets. The arrangement produces two separate electrostatic fields, one between the nozzle and the extractor electrode and the second between the extractor electrode and the walls of the scrubber. Due to the arrangement of the two electrostatic fields, the droplets must traverse a longer path before they encounter the walls.
Still another representative of the prior art is the German Pat. No. 2628822, issued July 25, 1913. Therein is disclosed a tube having pin holes through which water issues in a stream which breaks up into a fog-like rain of droplets. High voltage applied to the droplets will charge the droplets which are attracted to collection plates having an opposite charge. A device of this type provides little control over the droplet size and does not provide for a maximum charge density on the droplet. Little or no attention is given to controlling the operating parameters to yield a highly efficient scrubber.
It is accordingly an object of the present invention to provide a method of removing particles and undesirable fluids from a gas stream which is characterized by greater performance efficiency and which requires less energy than prior art methods.
A further object of the invention is to provide a method of the type discussed which will remove particles within a wide range of sizes by the simple adjustment of the operating parameters such as droplet size, magnitude of the electrostatic field and the like.
Another object of the present invention is to provide a method for removing particles from a gas stream in which the removal efficiency is substantially independent of the resistivity of the particles.
Still a further object of the invention is to form droplets by electrohydrodynamic action without requiring a special extractor plate.